Treatment of flue gases

The reaction mechanism in the irradiated flue gas is probably quite complex, but basically the EB excites the gas molecules and promotes reactions that convert the oxides to acids. These then react with ammonia or calcium compounds to give solid products that are removed by the filter. The initiation reaction is believed to be brought about by radical formation, such as OH, 

or H02, in the irradiated flue gas. The reactions with the oxides may be given as follows  

Certain other reactions are also possible, some of which do not give solid products. For example  

Of the major components of flue gas, oxygen and water vapor influence the reactions of the oxides considerably, hut carbon dioxide does not. Under irradiation, NO is oxidized by reactions with O, OH, and H02 radicals. The resultant N02 is oxidized to HN03 by reaction with OH radicals. S02 is similarly oxidized to H2S04 by reactions with O and OH. The products can also be converted to aerosols and collected in electrostatic precipitators. 

---

Fig. 1 also shows the chemical reaction mechanism of 80, and NO c in air for producing acid rain [2]. The radicals (O, OH, and HO2) produced in air play an important role in oxidizing 80, and to produce sulfuric acid and nitric acid, which are the main components of acid rain. 8ome of these reactions also occur in a reaction chamber of electron beam treatment of flue gas. 

The energy, or power, of electron beam induced in the flue gas is divided and absorbed by their gas components roughly depending on their electron fraction. Therefore almost all the energy is absorbed by the main components of the flue gas, namely, N2, O2, CO2, and H2O. Table 2 shows a typical concentration of the components in coal-fired flue gas in Japan. The ratio of the total number of electrons in each gas components is also listed in the same table. The energy absorbed directly by the toxic components (SO2 and NO) is negligibly small. For electron beam treatment of flue gas, ammonia gas is added to the flue gas before the irradiation. The amount of ammonia is usually set as stoichiometrically, i.e., 2A[S02] + A[NO], where A[S02] and A[NO] are the concentrations of SO2 and NO intended to be treated, respectively. The concentration of ammonia is usually higher than the initial concentration of SO2 and NO however, it is still far lower than that of the main components. 

These positive ions are the main source of the OH radical, which is one of the most important radicals for electron beam treatment of flue gas as will be discussed in the next section. 

Carbon dioxide removal by reactive absorption in amine solutions is also applied on the commercial scale, for instance, in the treatment of flue gas (see later in this chapter). Another possible application field of the technique is gas desulfurization, in which H2S is removed and converted to sulfur by means of reactive absorption. Aqueous solutions of ferric chelates (160-162) as well as tetramethylene sulfone, pyridine, quinoline, and polyglycol ether solutions of S02 (163,164) have been proposed as solvents. Reactive absorption can also be used for NOx reduction and removal from flue or exhaust gases (165,166). The separation of light olefins and paraffins by means of a reversible chemical com-plexation of olefins with Ag(I) or Cu(I) compounds in aqueous and nonaqueous solutions is another very interesting example of reactive absorption, one that could possibly replace the conventional cryogenic distillation technology (167). 

Developing a biological CO2 fixation system for culture ponds in salty water based on the treatment of flue gas from town s utility located on the Black Sea coast would provide the opportunity of using the algae crop as additional fuel to the power station. Advantages are offered by the significant solar radiation that provides a notable daily rate of CO2 fixation. 

Altpeter, E. Wallis, E. Heubner, U. Erprobung verschiedener metallischer Legierungen fur die Abwasseraufbereitung hinter Rauchgasentschwefelimgsanlagen (Evaluation of different metal alloys for waste water treatment of flue gas desulphurisation plants) (in German) Werkst. Korros. 45 (1994) 10, p. 539-549... 

Treatment of Flue Gas from Power Plants (Overview)... 

Treatment of flue gas from power plants involves three steps separation of fly ash, which is collected by an electrostatic precipitator, catalytic reduction of NO and SO2 separation by scrubbing using a slurry of a sorbent, usually limestone or lime. 

In the combustion area, heavy slag and ash may form, preventing the passage of flue gas and blinding tubes. Locations should be precisely noted to provide fireside adjustments or to implement a fuel treatment program. In coal-fired boilers, drums, tubes, and headers should be inspected for abrasion from clinker and fly ash. 

In any case, whether emissions are wet or dry, the presence of soot in the flue gas effectively means lost Btus. Where fuel treatments are employed, their function is to limit the supply side of flue gas emissions by improving the combustion process. 

The flue gas from municipal waste incinerator boilers contains SO2, and HCl. To remove these harmful components simultaneously by dry process, electron beam treatment method was investigated. The pilot-scale test was conducted in Matsudo, Japan, in 1992 with a flue gas of 1000 m /hr [34]. Recently, dioxins, namely, poly-chlorinated-di-benzo-paradioxins (PCDDs) and poly-chrorinated-di-benzo-furan (PCDFs), from incinerators have become a very serious problem because of their high toxicity. Pilot-scale tests to decompose dioxins by electron beam irradiation were conducted in Karlsruhe, Germany [35], and in Takahama, Japan [36], using almost the same capacity of flue gas, 1000 m /hr. Very promising results were obtained with decomposing more than 90% of dioxins. 

Research Strategy for the Development of Flue Gas Treatment Technology... 

Satriana (2) provides a summary of the development of flue gas treatment technology. The first commercial application of flue gas scrubbing for sulfur dioxide control was at the Battersea-A Power Station [228 MW(e)] in London, England, in 1933. The process used a packed spray tower with a tail-end alkaline wash to remove 90 percent of the sulfur dioxide and particulates. Alkaline water from the Thames River provided most of the alkali for absorption. The scrubber effluent was discharged back into the Thames River after oxidation and settling. A similar process was also operated at the Battersea-B Power Station [245 MW(e)] beginning in 1949. The Battersea-B system operated successfully until 1969, when desulfurization efforts were suspended due to adverse effects on Thames River water quality. The Battersea-A system continued until 1975, when the station was closed. 

Capital Equipment Costs The initial investment made by a utility to control pollutant emissions is in the equipment comprising the flue gas treatment system. The amount of the investment in this equipment purchase is directly related to the size and complexity of the equipment itself. The vessels required in this application must be large because of the volume of flue gas that must be treated. Some equipment, such as spray dryers or absorbers, can be 20 to 50 feet in diameter, and spray towers can be over 100 feet in height. Considerable preparation must go into sizing these vessels properly in order to maximize gas-sorbent contact time and minimize scaling and other operational problems. Much of this equipment, especially in wet systems, must be constructed of costly corrosion-resistant materials. Elaborate valving systems must be set up to control flows to the system. 

Restrictions Placed on the Boiler and Its Operation Many of the flue gas treatment system elements already discussed place restrictions on the boiler and its operation. Some examples of flue gas treatment system requirements that could restrict boiler operations would be the necessity for flue gas reheat, the ability of the flue gas treatment system to follow boiler load, the increased dust loading of dry systems, and the increased size and complexity of the system. In addition, many systems require a specific flue gas temperature for proper operation. This temperature requirement determines the location where the flue gas is subjected to treatment, thus affecting the design of downstream... 

Sometimes the opposite is also the case, that is, the boiler itself restricts the type of flue gas treatment system that can be installed. Most electric power generating facilities have coal specifications required by the boiler design and the manufacturer s warranty. These sulfur and ash specifications are designed to optimize boiler performance and avoid fatigue of boiler materials. Boiler design and coal specifications can limit the type of system that can be applied. The construction date of a boiler determines the level of pollutant control required, and it also goes far toward determining the type of flue gas treatment system that can achieve this level of emissions control. 

A related area of research would include efforts to reduce the water requirements of flue gas treatment systems. This would be especially important to the wet throwaway systems most commonly used today. Research to improve the dewatering properties of scrubber sludge could be combined with the development of techniques to separate spent and unused sorbent, thus reducing both water and sorbent consumption while also reducing the volume and mass of solid waste to be handled. The use of waste or recycled water in flue gas treatment systems would also reduce water needs. Improved sludge stabilization methods that also remove water could also lower costs associated with both waste disposal and water consumption. 

Other research areas that could improve the effectiveness and economics of flue gas treatment are (1) the development of better instrumentation for system control and (2) the evaluation of superior corrosion-resistant materials. System optimization cannot occur without a good means of system control. Many existing flue gas treatment systems are particularly sensitive to instrument feedback and response. The ability to control closely the operation of these systems could enhance their effectiveness and reliability. The presence of water in flue gas treatment systems... 

---